Weak Bonds Joint Effects Catalyze the Cleavage of Strong C?C Bond of Lignin-Inspired Compounds and Lignin in Air by Ionic Liquids

Article abstract of DOI:10.1002/cssc.202001828

Oxidation of lignin to value-added aromatics through selective C?C bond cleavage via metal-free and mild strategies is promising but challenging. It was discovered that the cations of ionic liquids (ILs) could effectively catalyze this kind of strong bond

Full text of DOI:10.1002/cssc.202001828

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Weak Bonds Joint Effects Catalyze the Cleavage of Strong
CÀ C Bond of Lignin-Inspired Compounds and Lignin in Air
by Ionic Liquids
Ying Kang,[a, b, c] Yongqing Yang,[a, b] Xiaoqian Yao,[a] Yanrong Liu,[d] Xiaoyan Ji,[d] Jiayu Xin,[a, b]
Junli Xu,[a] Huixian Dong,[a] Dongxia Yan,[a] Hongyan He,[a] and Xingmei Lu*[a, b, c]
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Oxidation of lignin to value-added aromatics through selective
CÀ C bond cleavage via metal-free and mild strategies is
promising but challenging. It was discovered that the cations of
ionic liquids (ILs) could effectively catalyze this kind of strong
bond cleavage by forming multiple weak hydrogen bonds,
enabling the reaction conducted in air at temperature lower
than 373 K without metal-containing catalysts. The cation
[CPMim]+ (1-propylronitrile-3-methylimidazolium) afforded the
highest efficiency in CÀ C bond cleavage, in which high yields
(>90%) of oxidative products were achieved. [CPMim]+ could
form three ipsilateral hydrogen bonds with the oxygen atom of
C=O and ether bonds at both sides of the CÀ C bond. The weak
bonds joint effects could promote adjacent CÀ H bond cleave to
form free radicals and thereby catalyze the fragmentation of
the strong CÀ C. This work opens up an eco-friendly and energy-
efficient route for direct valorization of lignin by enhancing IL
properties via tuning the cation.
Introduction
alternative to fossil fuels is highly meaningful but challeng-
ing for sustainable development of our society.
Lignin is an abundant renewable resource. It primarily
consists of methoxylated phenylpropanoid subunits and
approximately accounts for 30 % of non-fossil organic
carbon in the biosphere.[1] Owing to its aromatic structure,
utilization of lignin is now considered as a promising
sustainable route to value-added chemicals,[2] liberating us
from the reliance on limited fossil fuels. To date, unfortu-
nately, lignin valorization still remains a great challenge due
to its highly cross-linked and recalcitrant structure. Hence,
efficient utilization of the renewable lignin source as an
Oxidation of lignin remains one of the most promising
methods for selectively cleaving lignin inter-unit linkages
like aryl ether bonds and carbon-carbon bonds, significantly
preserving the lignin aromatic structure.[3] Among all the
oxidants, H2O2 and air have shown great promise for the
advantages of being green, low-cost, and accessible.[4] In
terms of mechanism, free radicals like hydroxyl radical are
generated and react with functional groups of lignin (for
example, the alcohol group) to harvest desired aromatic
products.[3a,4a] For H2O2, however, efforts are still necessary
regarding safety in handling due to its instability. In this
respect, much attention has been paid to the usage of air as
oxidant to achieve lignin depolymerization, in which metal-
based catalysts are usually applied involving polyoxometa-
lates or transition metals.[5] Nevertheless, the implementa-
tion of metal-based catalysts usually gives rise to issues like
the limitation of metal resources and leaching of metals.
Hence, the exploration of metal-free oxidative routes using
air as an oxidant is necessary. Recent studies have reported
microwave-assisted catalyst-free oxidation of lignin in air at
[a] Dr. Y. Kang, Dr. Y. Yang, Prof. Dr. X. Yao, Prof. Dr. J. Xin, Prof. Dr. J. Xu,
H. Dong, Prof. Dr. D. Yan, Prof. Dr. H. He, Prof. Dr. X. Lu
Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of
Green Process and Engineering
State Key Laboratory of Multiphase Complex Systems, Institute of Process
Engineering
Chinese Academy of Sciences
Beijing, 100190 (P. R. China)
E-mail: xmlu@ipe.ac.cn
[b] Dr. Y. Kang, Dr. Y. Yang, Prof. Dr. J. Xin, Prof. Dr. X. Lu
School of Chemistry and Chemical Engineering
University of Chinese Academy of Sciences
Beijing, 100190 (P. R. China)
°
elevated temperature (170 or 180 C), and the valuable
products like vanillin could be obtained effectively.[6] In
spite of the encouraging results, relatively harsh conditions
are needed to cleave inert lignin bonds. Obviously, current
oxidation of lignin in air is generally associated with metal-
based catalysts and/or relatively harsh conditions, where
free radicals could be formed to oxidize lignin.
[c] Dr. Y. Kang, Prof. Dr. X. Lu
Innovation Academy for Green Manufacture
Chinese Academy of Sciences
Beijing, 100190 (P. R. China)
[d] Prof. Dr. Y. Liu, Prof. Dr. X. Ji
Energy Engineering, Division of Energy Science
Luleå University of Technology
Luleå, 97187 (Sweden)
Hence, it is highly desirable to develop safe, mild, and
metal-free routes for lignin valorization by air. But how to
meet these requirements? Fortunately, the green media
ionic liquids (ILs) have been proven to be promising metal-
free media for lignin upgrading not only as solvent[7] but
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202001828
This publication is part of a Special Issue focusing on “Lignin Valorization:
From Theory to Practice”. Please visit the issue at bit.ly/cscLignin.
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also as effective catalysts to assist in fragmenting lignin
network.[8] Note that both cations and anions of ILs could
catalyze substrates through interactions like hydrogen bond
formation;[9] it may accordingly give access to effective
lignin oxidation under milder conditions by adjusting
appropriate IL components. The role of anions on lignin
depolymerization has been extensively investigated:[10] the
lignin-inspired compound 2-phenoxyacetophenone could
be oxidized into benzoic acid and phenol in ILs containing
the anion [NTf2]À . Actually, cations of ILs could also activate
substrates by forming hydrogen bonds and exert efficient
promotion.[11] Hence, it would be very attractive to explore
the interaction of IL cations with lignin inter-unit linkages in
depth, as well as the role of the interaction in further lignin
valorization.
Notably, the non-negligible role of imidazolium-based IL
cations in promoting the ether bond cleavage has been
reported during cellobiose depolymerization.[12] One of the
key steps is the formation of multiple hydrogen bonds
between cation and three ipsilateral hydroxy groups at both
sides of the ether bond in cellulose, which could contribute
to the following CÀ O bond fragmentation. As plenty of
ether linkages exist in the lignin structure,[13] the discovery
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Scheme 1. Structure of [CPMim][NTf2].
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catalyst and harsh conditions are avoided. The cleavage of
the CαÀ Cβ bond was effectively catalyzed by the functional-
ized cation, which could form three ipsilateral hydrogen
bonds with the substrates at both sides of the CαÀ Cβ bond.
Affected by the WBJE, the CβÀ H bond was activated to yield
free radicals and initiated the following CαÀ Cβ bond
fragmentation to produce desired aromatics. In addition,
alkali lignin could also be effectively depolymerized in
[CPMim][NTf2]. The strategy proposes an alternative route
for the depolymerization of lignin into valuable aromatics
under safe, metal-free, and mild conditions.
would propose
a guidance for lignin valorization by
selective bond cleavage. Interestingly, for lignin, multiple
hydrogen bonds have been reported to simultaneously
occur between H atoms of imidazolium-based cations and O
atoms of lignin fragments.[14] Although the theoretical
analysis mainly focused on the lignin dissolution mechanism
in ILs, the interaction may contribute to further bond
cleavage. The hydrogen-bonding interactions of an imidazo-
lium-based cation could be affected by the incorporation of
a functional group.[15] In particular, the nitrile group is
electron-rich, which makes it liable to form hydrogen
bonds.[15,16] Introducing a nitrile group could cause an
enhancement in hydrogen-bonding interactions of IL cati-
ons. Meanwhile, upon the incorporation of a CN group,
conformational changes in the imidazolium group occur,
leading to remarkable changes in IL properties such as
higher stability and decrease in toxicity.[15] Hence, the
application of the ILs containing nitrile-functionalized cati-
ons has great potential in lignin valorization. Due to the
enhanced hydrogen-bonding interactions caused by this
kind of cation, it would be possible for the cation to form
multiple ipsilateral hydrogen bonds at both sides of the
targeted lignin linkages simultaneously. Influenced by weak
bonds joint effects (WBJE) of the hydrogen bonds, the
strong lignin linkages might be cleaved under mild con-
ditions, contributing to the effective and mild lignin
depolymerization, as well as the expanded valorization of
other high-molecular polymers, through a green, safe, and
sustainable approach.
Results and Discussion
Influence of IL structure on the oxidation of lignin-inspired
aryl ethers
Given the complexity of lignin, we first conducted the
oxidation of the β-O-4 lignin-inspired aryl ether, that is, 2-
phenoxyacetophenone (1), in air using various ILs. Several
neutral ILs with different anions and cations were used, and
their structures were shown in Scheme S1. Consistent with
previous studies,[10a,b] the IL containing bis((trifluoromethyl)
sulfonyl) imide [NTf2]À was found to exhibit high perform-
ance compared to other anions including trifluoromethane-
sulfonate [OTf]À , tetrafluoroborate [BF4]À , hexafluorophos-
phate [PF6]À , and nitrate [NO3]À in the reaction (Table 1,
entries 1–5). This might be attributed to the strong electro-
negativity of [NTf2]À , which could be in favor of the cleavage
Table 1. The influence of IL’s structure on the oxidation.[a]
Entry
IL
Conversion
[%]
Benzoic acid
[%]
Phenol
[%]
1
2
3
4
5
6
7
8
9
[BMim][OTf]
[BMim][BF4]
[BMim][PF6]
[BMim][NO3]
[BMim][NTf2]
[CPMim][NTf2]
[EMim][NTf2]
[PMim][NTf2]
[AMim][NTf2]
10.9
22.1
24.3
23.4
64.2
83.7
94.2
74.9
34.4
2.5
2.6
1.3
3.9
44.4
78.2
85.2
54.1
3.3
6.8
0.7
0.6
1.1
28.0
72.4
57.0
49.7
5.1
Based on the above consideration, we present an
efficient strategy for lignin oxidation catalyzed by the IL
[CPMim][NTf2] (as shown in Scheme 1; [CPMim]+ = 1-propyl-
ronitrile-3-methylimidazolium) with the nitrile-functional-
ized imidazole cation in air, through which the metal-based
[a] Reaction conditions: 1 (10 mg), IL (500 mg), [PrHSO3Mim][OTf] (2 wt%),
H2O (1 wt%), atmosphere, 353 K, 90 min.
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of 1 to produce benzoic acid and phenol, as reported in
previous work.[10a] In addition, influence of the IL cation on
the oxidative reaction was also investigated by varying the
alkyl chain of the imidazolium ring, including 1-propylroni-
trile-3-methylimidazolium cation [CPMim]+, 1-ethyl-3-meth-
spectrophotometrically determined and calculated as listed
in Table S1 by UV/Vis spectroscopy (Figure S1).[10b,18] The
results indicated that the addition of Brønsted acid was
crucial. In the presence of BAIL, the depolymerization was
preferred under higher acid strength. [PrSO3HMim][OTf]
with the highest Brønsted acidity achieved the best
performance, while an obvious trend towards lower con-
version and product yields was observed with gradually
decreasing acidity of BAILs. A similar trend was observed for
mineral acids, and H2SO4 with higher acidity exhibited better
performance compared with H3PO4.
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ylimidazolium
cation
[EMim]+,
1-propenyl-3-meth-
ylimidazolium cation [PMim]+, 1-butyl-3-methylimidazolium
cation [BMim]+, and 1-allyl-3-methylimidazolium cation
[AMim]+. As illustrated in Table 1 (entries 5–9), it was found
that the cation of the IL has a great influence on the
depolymerization of 1. For the IL cation with the nitrile
group, [CPMim]+, the highest selectivity of products was
obtained: 83.7 % of 1 was converted to benzoic acid and
phenol with yields of 78.2 and 72.4 %, respectively. For
comparison, lower activity was exerted in IL with [PMim]+.
Meanwhile, the length of alkyl chain in the IL cation was
also effective for this reaction; the longer alkyl chain could
exert lower performance in the CαÀ Cβ bond cleavage of 1.
One possible reason is that longer alkyl chain leads to an
increase in steric hindrance, hampering the interaction
between cation and 1.[17] In addition, the IL with the alkene-
functionalized cation, [AMim]+, exerted the lowest perform-
ance among the ILs used, affording the products with yields
of lower than 6 %. We could deduce from the results that
the cation [CPMim]+ could improve the reaction efficiency.
It might be relevant to the fact that [CPMim]+ could form H-
bonds with 1 and consequently catalyze 1 to cleave bonds,
which can be proved by the NMR spectroscopy, Fourier-
transform (FT)IR spectroscopy, and DFT calculations below.
Furthermore, the influence of Brønsted acid on the
reaction was investigated by adding a series of SO3H-
functionalized imidazolium-based Brønsted acid ILs (BAILs)
(Scheme S2) and the commonly used classical mineral acids
(H2SO4, H3PO4) to [CPMim][NTf2], as shown in Figure 1. The
Hammett acidity functions (H0) of the Brønsted acids were
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Effects of reaction parameters and recycle experiments
The effects of various reaction parameters on the oxidation
were studied with results listed in Table S2 and shown in
Figure 2. As expected, atmosphere is crucial for the reaction.
High conversion of 1 and product yields could be achieved
in air, while replacing air by argon led to great decrease of
conversion (Table S2). Meanwhile, the reaction conducted in
oxygen afford high conversion of 1, indicating the oxidative
effect of the oxygen. However, the product yield in oxygen
was slightly lower than that in air, which might be
attributed to the further oxidation in oxygen. In addition,
we investigated the O2 absorption performance of [CPMim]
[NTf2] under 353 K and atmospheric pressure. As shown in
Figure S2, the O2 absorption capacity of [CPMim][NTf2] is as
low as 0.75 mg O2 gÀ 1 IL at 353 K and atmospheric pressure.
The solubility of O2 in ILs has been investigated, and
comparable low O2 absorption capacity was measured in
previous work.[19] The low O2 absorption capacity of [CPMim]
[NTf2] indicates that the oxidative process might mainly
occur at the interface between the reaction system and air.
Meanwhile, we also investigated the influence of water. As
shown in Figure 2a, it was found that water could signifi-
cantly promote the reaction. The conversion and product
yields display an obvious trend towards higher value with
increasing water addition from 0 to 1 wt %, and a 1 wt %
addition of water afforded the best product yields. In
addition, effect of [PrSO3HMim][OTf] amount on the reaction
was studied (Figure 2b). The increase of [PrSO3HMim][OTf]
addition from
0 to 3 wt % significantly enhanced the
reaction with an obvious increase of conversion from 0 to
91.9 %. Further increasing [PrSO3HMim][OTf] content over
3 wt % led to a decrease in product yields, which might be
caused by the side reactions in the presence of excessive
BAIL.[20]
The effect of time on the reaction at 353, 363, and 373 K
was further evaluated, as illustrated in Figure 2c–e. On the
one hand, high temperature was beneficial to accelerate the
reaction. At 353 K, 98.3 % of 1 gradually converted into
benzoic acid and phenol with yields of 91.7 and 76.8 % after
120 min, while the times for achieving conversion over 98 %
were 90 min at 363 K and 50 min at 373 K, indicating the
enhanced reaction rate with temperature. On the other
hand, enhancing reaction temperature led to the slight
Figure 1. Relationship among acidity H0, yields of products and conversion.
Conditions: 1 (10 mg), [CPMim][NTf2] (500 mg), c[H+] (0.028 mmol), H2O
(1 wt%), atmosphere, 353 K, 90 min.
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Figure 2. Effects of reaction parameters with the conditions: 1 (10 mg), [CPMim][NTf2] (500 mg), atmosphere, 353 K, varying the amounts of H2O (a),
[PrSO3HMim][OTf] (b), 353 K (c), 363 K (d), 373 K (e); the reuse of [CPMim][NTf2] at the conditions: 1 (10 mg), [CPMim][NTf2] (500 mg), [PrHSO3Mim][OTf]
(3 wt%), H2O (1 wt%), atmosphere, 353 K, 120 min (f).
decline in product yields, which might be attributed to the
side reaction like repolymerization at high temperature.[20]
In addition, we examined the recyclability of [CPMim]
[NTf2] as illustrated in Figure 2f. The reuse of [CPMim][NTf2]
was accomplished after the full extraction of aromatic
products. The performance of the IL was not greatly affected
by recycling, and the yields of products and conversion only
slightly decreased in the six additional runs, confirming that
[CPMim][NTf2] was stable under the reaction conditions and
could be reused.
(TEMPO).[21] Almost no products were detected by adding
1.5 wt % TEMPO into the reactor (Figure S3), indicating that
the oxidation reaction might proceed via a free radical
process.
Although the mechanism for 1 depolymerization cata-
lyzed by the anion [NTf2]À has been studied in our and other
research groups’ previous work,[10a,b] the catalytic mecha-
nism of the IL cation is still unclear. It is well documented in
the literature that imidazolium cations of ILs tend to form
hydrogen bonds with oxygen atoms of lignin-inspired aryl
ethers through the C2À H of the imidazolium ring.[14b] Herein,
a plausible mechanism of the WBJE of three ipsilateral
hydrogen bonds to catalyze the strong CαÀ Cβ bond cleavage
through free radical pathway was proposed, on the basis of
the experimental results, the NMR and FTIR spectroscopic
analysis, the knowledge from previous studies, and the DFT
calculations, as illustrated in Scheme 2.
Mechanism of the oxidation
To gain insights on the reaction pathway, an additional
experiment was performed in the presence of the free
radical
scavenger
2,2,6,6-tetramethyl-piperidinyloxy
Scheme 2. Possible mechanism of oxidation of 1.
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Figure 3. 1H NMR spectra of [CPMim][NTf2] and its mixture with 1 with molar ratio of 2:1.
First, the CβÀ H bond of 1 was cleaved by the interactions
of [CPMim][NTf2] with 1 in the presence of O2 in air,
affording the alkyl radical (denoted as “a” in Scheme 2). On
the one hand, oxygen could react with CβÀ H of 1 and lead
to a charge transfer.[22] On the other hand, three ipsilateral
hydrogen bonds formed between the cation [CPMim]+ and
1, by which the WBJE could make CβÀ H labile to cleave.
Meanwhile, the anion [NTf2]À could also contribute to the
delocalization of electrons of CβÀ H.[10a] After the reaction
between free radicals a and O2, the peroxy radical b formed
and could abstract protons from 1,[23] in which the CβÀ H was
labile to cleavage by the WBJE with [CPMim]+, to afford the
free radical a and an intermediate hydroperoxide. The
hydroperoxide was attacked by H+ and H2O to generate a
hydrated ketone. Then the hydrated ketone experienced the
cleavage of the CαÀ Cβ bond and produced benzoic acid and
phenyl formate, which subsequently hydrolyzed to formic
acid and phenol.
change in its environment. As the H atom of methyl in
imidazolium cation has been reported to be involved in
hydrogen bonding,[12b,26] we speculated that the downfield
shift of the C6À H signal might be relevant to the formation
of hydrogen bonds. To further investigate the interactions
between ILs and 1, FTIR analysis was conducted as shown in
Figure 4. The red-shifted bands of the C2À H move from 3121
to 3110 cmÀ 1 and the C6À H move from 2968 to 2956 cmÀ 1
upon addition of 1,[15,24] as well as the broadening of these
two peaks, possibly indicating the formation of hydrogen
bonds between [CPMim]+ and 1.[27] Meanwhile, after mixing
1 with IL, a red shift of the C=O group of 1 was observed
from 1708 to 1698 cmÀ 1, and the signal for the CÀ OÀ C group
at 1250 cmÀ 1 was not observed, which might be involved in
As mentioned, during the initial stage of oxidation of 1,
the cleavage of the CβÀ H bond occurs by the interactions
between ILs and 1. Previous work discussed the effect of the
anion [NTf2]À on the delocalization of electrons of the CβÀ H
bond, which could facilitate its cleavage.[10a] To further
identify the role of the cation [CPMim]+, 1H NMR spectra
were used to study the interactions between [CPMim]+ and
1. Due to the formation of the electron-deficient C=N π
bond, C-2 on the imidazolium ring is positive,[24] and the
C2À H is anticipated to form hydrogen bonds with substrates.
As shown in Figure 3, the C2À H signal of [CPMim]+ at
8.45 ppm shifts to 8.53 ppm with the addition of 1 to
[CPMim][NTf2].[16] The downfield shift is mainly attributed to
the formation of hydrogen bonds.[25] Interestingly, the
downfield shift of the C6À H signal in [CPMim]+ is also
observed to move from 3.78 to 3.86 ppm, indicating a
Figure 4. FTIR spectra of 1, [CPMim][NTf2], and its mixture with 1 with molar
ratio of 1:2.
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the red shift and be included in the broad peak around
1196 cmÀ 1 [2a,20]
The changes imply that these two groups of
1 might be involved in hydrogen bonding. To confirm the
above inference and investigate the role of the cation
[CPMim]+ in promoting the strong CαÀ Cβ bond cleavage,
DFT calculations were subsequently implemented.
Since formic acid was not detected in the products, it
might be converted to CO2 under heating.[10a] To verify the
conjecture, an FTIR spectrum of the reaction was obtained
as shown in Figure S4. Compared to the spectrum without 1,
the one with 1 showed a new peak at 2322 cmÀ 2 typical for
CO2, indicating the conversion of formic acid.
1
2
3
4
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8
9
.
We analyzed and compared the interaction structures of
1 with [CPMim]+. Figure 5 showed the positions with the
lowest energy in which [CPMim]+ located around 1. As
expected, the [CPMim]+ could form three ipsilateral hydro-
gen bonds at both sides of the CαÀ Cβ bond with the oxygen
of Cα=O and ether bond simultaneously; the interaction
Oxidation of other lignin-inspired aryl ethers
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To verify the applicability of [CPMim][NTf2], another β-O-4
lignin-inspired aryl ether, 2-(2-methoxyphenoxy)-1-(4-meth-
oxyphenyl) ethanone (2), was depolymerized in the IL
system in air, as shown in Scheme 3. Under similar con-
ditions as presented in Figure 2f, 98.5 % conversion of 2
could be achieved, affording 94.1 % yield of anisic acid and
76.8 % yield of guaiacol, respectively. The results indicated
that [CPMim][NTf2] had good performance in catalyzing the
CαÀ Cβ cleavage of the β-O-4 ether linkage in lignin.
energy is 76.1 kJ molÀ 1
. Affected by the WBJE of the
interactions, the adjacent CβÀ H bond could be labile to
cleavage. By calculation, the changes in the lengths and the
bond dissociation energy of CβÀ H bond in 1 were observed
(Table 2). One of the CβÀ H bonds, denoted as CβÀ H2, was
activated. The bond length of CβÀ H2 has been elongated
from 1.092 to 1.094 Å. In addition, the bond dissociation
energy was affected and decreased from 335.0 to
310.7 kJ molÀ 1. As a result, influenced by the multiple hydro-
gen bonds, the CβÀ H2 has been activated and is more
susceptible to cleave to generate free radicals. For compar-
ison, the interaction between 1 and cation [PMim]+ with
unfunctionalized side alkyl chain was also surveyed, and the
conformer with the lowest energy was observed (Figure 5).
There are also three ipsilateral hydrogen bonds forming
between [PMim]+ and 1, leading to the bond length of
CβÀ H2 elongated from 1.092 to 1.098 Å. However, compared
with the results at the presence of [CPMim]+, the interaction
energy is lower, and the bond dissociation energy of CβÀ H2
show a higher value (326.1 kJ molÀ 1), indicating that CβÀ H2
was more susceptible to be activated by [CPMim]+, which is
in accord with the above experimental results.
Oxidation of alkali lignin
Subsequently, the depolymerization of alkali lignin was
studied. FTIR spectroscopy, 2D heteronuclear single quan-
tum coherence (HSQC) NMR spectroscopy, and GC-MS
analysis were carried out to verify the oxidative process of
alkali lignin. As shown in Figure S5, the alkali lignin and the
residual lignin had similar core structures by comparing the
absorption bands in FTIR spectra (Table S3), while several
changes were presented for the residual lignin. The
disappearance of the band at 1170 cmÀ 1
, which was
attributed to the CÀ O stretching in ester groups, demon-
strated the cleavage of benzyl esters. Meanwhile, the
observed absorption band at 1660 cmÀ 1 in the residue lignin
was assigned to C=O stretching in conjugated ketone,
indicating that the alkali lignin could be oxidized during the
oxidation process.
To further investigate the oxidation process, HSQC
analysis was conducted. The δC/δH region of 50–95/2.5–
6 ppm in the HSQC spectra represents the oxygenated side-
chain structure, and the corresponding lignin subunits in
the region were assigned by comparing with previous
work.[28] Strong signals of aryl methoxy groups were
observed in both original alkali lignin and the oxidized
lignin spectra (Figure 6), with the signal at δC/δH 56.3/
Figure 5. Optimized interaction structures of 1 with [CPMim]+ and [PMim]+
by B3LYP/6-311+G(d,p), and the changes of interaction energy (E) for 1
while it interacts with [CPMim]+ and [PMim]+.
3.7 ppm. The original lignin spectrum demonstrated
a
characteristic lignin containing the β-O-4 (A) cross-peaks.
Table 2. Changes of bond length and bond dissociation energy for CÀ H
bonds in 1 while it interacts with [CPMim]+.
Bond length [Å]
Bond dissociation energy [kJmolÀ 1
]
CβÀ H2
CβÀ H1
CβÀ H2
1
1.100
1.092
1.094
1.098
335.0
310.7
326.1
1 with [CPMim]+ 1.102
1 with [PMim]+
1.096
Scheme 3. Oxidation depolymerization of lignin-inspired aryl ether 2.
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Figure 6. HSQC NMR spectra [ppm] of original alkali lignin, oxidized lignin, and oily products samples. Reaction conditions were the same as those of
Figure 2f.
The signals observed at δC/δH 72.1/4.8, 86.5/4.1, and 60.5/
3.6 ppm assigned to CαÀ Hα, CβÀ Hβ, and CγÀ Hγ correlations,
respectively. Meanwhile, the signals from β-5 (B) and β-β (C)
substructures were observed in original alkali lignin. Regard-
ing the β-5 (B) substructure, the CαÀ Hα, CβÀ Hβ, and CγÀ Hγ
correlations corresponded to δC/δH 88.2/5.6, 54.7/3.5, and
GC-MS structure analysis was conducted as shown in
Figure S6, the aromatic products further confirmed the
oxidative process of alkali lignin through bond cleavage in
[CPMim][NTf2].
62.9/3.7 ppm, respectively, and the signal for CγÀ Hγ of β-β Conclusions
(C) substructure was observed at δC/δH 69.8/3.6 ppm.
Instead, for the oxidized lignin, the corresponding signals
for A, B, and C substructures are absent or show low
intensity, manifesting that most of the original linkages, like
the β-O-4 bond, are fragmented in the IL system during the
oxidation process. Additionally, in the aromatic region, the
signals for ‘G’ and ‘S’ units were observed in both the
original and oxidized alkali lignin spectra. For the oily
products extracted by methyl tert-butyl ether (MTBE), the
In this work, we demonstrate a safe, effective, and green
method for oxidative CÀ C bond cleavage of β-O-4 lignin-
inspired aryl ethers in air catalyzed by the weak bonds joint
effects (WBJE) involving the nitrile-functionalized cation of
ionic liquids (ILs) at 353 K under metal-free conditions.
Noteworthily, three ipsilateral hydrogen bonds could simul-
taneously form at both sides of targeted CαÀ Cβ bond among
CN group-functionalized imidazolium-based cations and the
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O atoms of Cα=O and ether bond. Upon the WBJE, the Acknowledgements
adjacent CβÀ H was activated and liable to cleave to form
1
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free radicals, following which the strong CαÀ Cβ bond was
selectively fragmented to yield corresponding aromatic
products with high yields. Furthermore, the alkali lignin
could also be effectively depolymerized. The method is
highly efficient, metal-free, safe in handling, and with low
energy usage. We believe that this study may shed light on
metal-free and mild bond fragmentations of the inert
linkages in the complex macromolecule with low energy
usage and high efficiency.
Thanks for the proposal and support of Professor Suojiang Zhang
to this study. This research was financially supported by the
National Key R&D Program of China (No. 2018YFB1501600),
National Natural Science Foundation of China (No. 21878292, No.
21606240, No. 21878314), K. C. Wong Education Foundation (No.
GJTD-2018-04), the Strategic Priority Research Program of Chinese
Academy of Science (No. XDA21060300) and China Postdoctoral
Science Foundation (2019M660797).
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Conflict of Interest
Experimental Section
The authors declare no conflict of interest.
Oxidation of alkali lignin and lignin-inspired aryl ethers: The
oxidation of lignin and lignin-inspired aryl ethers was carried out
under atmosphere, oxygen, or Ar in a glass reaction vial placed in a
heating block equipped with a mechanical stirrer. The reactor was
charged with a certain amount of ILs, BAILs, 1/alkali lignin, and
deionized water and exposed to atmosphere, oxygen or Ar. The
mixture was then heated to a desired temperature, and the
oxidation of lignin was allowed to proceed for a suitable time with
a stirring of 1000 rpm. When the reaction was complete, the
reactants were cooled to the room temperature. MTBE was added
to the reaction mixture to extract the products and the uncon-
verted 1.
Keywords: hydrogen bond · ionic liquids · lignin · oxidation ·
weak bonds joint effects
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The recycle experiments were conducted as follows: after the
products were extracted by MTBE from the reaction mixture, the IL
was washed by deionized water (3×5 mL) and dried at 343 K in
vacuum for 24 h. Then the IL was used in the next run.
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yield of products were calculated as shown in Equations (1) and (2):
Conversion ½%� ¼ ðC0 À CligninÞ � 100=C0
(1)
Yield ½%� ¼ Cproducts � 100=C0
(2)
where C0 is the initial concentration of lignin-inspired aryl ethers,
Clignin is the concentration of lignin-inspired aryl ethers at sampling
time, and Cproducts is the concentration of the products.
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extracted from the reaction mixture by MTBE, a 1:3 water/ethanol
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Manuscript received: July 30, 2020
Revised manuscript received: August 20, 2020
Accepted manuscript online: September 10, 2020
Version of record online: ■■■, ■■■■
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Two heads are better than one:
Nitrile-functionalized cations of ionic
liquids can form three ipsilateral
hydrogen bonds with oxygen atoms
of lignin-inspired compounds at both
sides of the CÀ C bond. The weak
bonds joint effects can catalyze the
CÀ C bond cleavage efficiently.
Dr. Y. Kang, Dr. Y. Yang, Prof. Dr. X.
Yao, Prof. Dr. Y. Liu, Prof. Dr. X. Ji,
Prof. Dr. J. Xin, Prof. Dr. J. Xu, H. Dong,
Prof. Dr. D. Yan, Prof. Dr. H. He,
Prof. Dr. X. Lu*
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Weak Bonds Joint Effects Catalyze
the Cleavage of Strong CÀ C Bond
of Lignin-Inspired Compounds and
Lignin in Air by Ionic Liquids